1. Field of the Invention
The present invention relates to apparatus for repairing insulation of an insulated conductor. More particularly, the present invention relates to such apparatus which is effective for repairing holes and bubbles in the insulation of an insulated conductor.
2. Description of the Prior Art
Insulated conductors, such as single conductors, coaxial cable, etc., are conventionally formed by extruding insulation about the conductive wire or wires. Typically, the insulation comprises a thermoplastic material. The insulated conductors are generally formed in lengths of 18,000 to 19,000 feet, lengths up to 37,000 feet being common. These lengths are typical in the manufacture of insulated conductors for use in sub-surface cables, etc.
The extrusion of insulator onto a conductor in the fabrication of an insulated conductor of such great length inevitably results in certain imperfections or defects in the extrusion. These show up, for example, as pinholes and bubbles, often caused by out-gasing during the extrusion process. These imperfections can not be tolerated in the finished insulated conductor and as a result, must be removed. Particularly where the insulated conductor is utilized in an underwafter environment, any hole in the insulation will allow water to contact the conductor, thereby totally eliminating the effectiveness of the system.
Pinholes and other imperfections in the extruded insulation are typically discovered by subjecting the insulated conductor to a spark test or other similar test. In the past, where the insulation of the conductor of many thousands feed in length was found to have two or more major defects, such as pinholes, the entire insulation on the conductor was stripped off and new insulation extruded on the conductor. This, of course, is an expensive process, is one which results in a substantial waste of materials, and is one which requires additional consumption of time and energy. Also, when re-extruding the insulation about the conductor, there is, of course, no guarantee that the new insulation will be sufficiently free of defects or imperfections to be successfully utilized.
Early attempts to repair insulation involved the simple taping of the area in which the insulation was worn or defective. This is still currently utilized where a temporary repair can be tolerated and where there is no requiement for the dimensions of the repaired insulated conductor to be no greater than the original dimensions of the insulated conductor. However, where dimensions are important, as in most applications, such temporary taping of the insulated conductor is not an acceptable repair method. Based upon these inadequacies, several types of apparatus and several methods have been proposed for the repair of defective conductor insulation.
One such method and apparatus is illustrated, for example, in U.S. Pat. No. 2,716,523 to Tator. The method described in this patent is adapted to the insulation of short lengths of insulated electric cables where the insulation has been worn away or where splices have been made. The insulation is applied to the electric conductor at a gap in the original insulation by fitting a mold around the exposed portion of the conductor, filling the mold with a mobile liquid plastisol at room temperature, heating the mold until the plastisol solidifies into a flexible solid and removing the cable from the mold so as to expose the plastisol to room temperature. The required use of a liquid plastisol somewhat complicates the insulation repair method and the molding apparatus has no acceptable provision for confining the heating which is carried out to the area of the repair. This can lead to a serious problem where the thermoplastic insulation outside of the molding zone is heated during the repair operation and becomes sufficiently fluid to damage the previusly undamaged portions of the insulated conductor. While this method is suggested as an improvement over a typical method of melting a thermoplastic material about the repair area of the insulated conductor, the method and apparatus of Tator has not been found to be commercially attractive.
A further method and apparatus for repairing damaged insulation is illustrated in U.S. Pat. No. 3,268,006 to Gill. The method and apparatus of this patent are specifically adapted to the repair of marine cables and the like. The exposed portion of the conductor to which a thermoplastic is to be applied is enclosed in a demountable mold having an extrusion chamber of a length shorter than the length of material to be introduced to repair the thermoplastic insulation. The mold chamber has in coaxial alignment an entrance die of variable diameter within a range including the inner and outer diameters, respectively of the thermoplastic layer to be applied and an exit die of a diameter which remains constant and is substantially equal to the external diameter of the thermoplastic layer. The conductor is located so that at the start of the repair operation, one end of the existing plastic layer is introduced into the exit die. Thermoplastic material is injected in the moltant condition into the mold chamber so that it unites with the thermoplastic layer at this one end and relative movement of the mold with respect to the conductor member longitudinally is carried out so that the successive portions of the conductor enter the entrance die and leave with the thermoplastic layer in position at the exit die. Generally, the mold unit is maintained stationary and to effect the repair, the cable is drawn through the mold chamber.
The method and apparatus of the Gill patent suffer from certain inherent disadvantages. The need to draw the conductor through the mold in order to repair insulation is a clear disadvantage, providing many more possibilities for inadequate repair. Moreover, like the molding apparatus illustrated in the Tator patent, no satisfactory provision is made in the mold of Gill to prevent the heating associated with the heated thermoplastic to go beyond the confines of the mold. Again, this type of system has not been found commercially attractive.
In areas different from insulation repair, but having some common features, various types of molds have been developed. For example, U.S. Pat. No. 3,496,609 to Ferguson illustrates an injection mold for vulcanizing the insulation of an electrical cable to a tapered metal flange mount. This mold includes a removable central section in the injection mold with the central section comprising a split, centrally grooved cable enclosing means for holding the cable and split centrally grooved spur enclosing means. Sets of graduated cable enclosing means and graduated spur enclosing means allow the mold to be utilized for a variety of different sizes of cables. This type of mold arrangement, however, like those previously discussed in connection with the Tator and Gill patents, does not provide for a cooling of the insulation material and, as such, can not be successfully adapted to the repair of the insulation of an insulated conductor.
U.S. Pat. No. 3,754,845 to Rauscher, discloses an apparatus for splicing jacketed cable in which a horizontally split mold is arranged to receive a cable which is to be insulated by extruding insulating material into the mold. The arrangement of the Rauscher patent includes tensioning blocks at opposite ends of the mold to maintain the cable centered during the molding operation. Heating element wires are arranged within the mold so that the jacket portions of insulation on the cable are not melted, but the center section of the mold is heated to a temperature above the melting point of the materials supplied by an extruder. Cooling water passages are provided for cooling the entire mold after the insulation operation.
Like the previously discussed apparatus, the apparatus used in splicing jacketed cable described in the Rauscher patent cannot be effectively utilized in the repair of insulation of an insulated conductor. Again, no adequate provision is made in this apparatus for preventing damage to the insulation during heating, particularly in an area adjacent the splicing zone.
U.S. Pat. No. 3,783,057 to McNerney, one of the present co-inventors, describes a wire insulation splicing technique for high temperature well logging cables. This patent illustrates a mold having enlarged heat sinks at the extremities of the mold. While this system is certainly quite effective in the splicing technique described, the presence of the enlarged heat sinks provides for a more cumbersome apparatus, not particularly adaptable to the repair of insulation of an insulated conductor. This mold, therefore, has not been applied to the repair of insulation. Accordingly, the art has still sought a machine or apparatus that can effectively repair the insulation of an insulated conductor in an efficient and effective manner, producing a repaired conductor having dimensions which do not exceed the original dimensions of the insulated conductor. This has now been achieved in accordance with the present invention.